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Apartment Block Load Assessment and Diversity
Dale Green
4 Posts
Just waiting for my copy of the IET Electrical Installation Design Guide to be be Delivered but in the meantime would be grateful of a bit of advice.
The guide as I understand it has a section on the topic  (section 3.5 pp36-42).
I have a some projects with apartment blocks, full apartment units, with their own services, kitchens and electric heating. Having carried out demand and diversity calculations for each apartment I want to look at the overall building diversity for DNO intake demand. Does the guide noted cover this? If it does could anyone advise what it does say and if not which other design guidance may cover this. The usual thing is that having carried out the apartment diversity the building demand always looks high when looking at buildings with 80+ apartments, so any assistance at this stage would help. Thanks
11 Replies
John Peckham
568 Posts
Yes it does it goes up to 100 units.
Dale Green
4 Posts
HI John
Looking at a few with differing numbers, but not as many as 100, but close at 98 on one?
465 Posts
I wouldn’t be holding my breath for enlightenment! Whilst there is some good stuff in the design guide, there is nothing new in the assessment of maximum demand. I was expecting a fresh approach with deliberations on EV charge points, heat pumps etc. Despite their impact on both installations and the DNO supply to installations, I don’t think they even get a mention in the book. 
465 Posts
Dale Green:
HI John
Looking at a few with differing numbers, but not as many as 100, but close at 98 on one?

Are you cabling from one central metering position?

Dale Green
4 Posts
Thanks for replies up to now.
Some will be from a single location (ones with fewer apartments) and the larger ones with metering locations on each floor.


That doesn't surprise me, the EV chargers allowance is something I have already raised given planning/building regulations changes and particularly having done many EV charger design installations in the last couple of years. The thing that always annoys me is the guidance within regulations, on site guide etc. has never aligned with DNO guidance. One can carry out a IET diversity calculation and come to a figure of say  12kW for a flat, so that gives the flat supply/cable, then a DNO say they allow around 3.5kW for a direct electrically heated flat as far as their network allowance is concerned when they have no idea what is in the flats, that's a diversity down to 30%. How does that ever stack up on Christmas Day, every oven full on for hours, heating on full wack etc.None of this is  ever going to change the flat supply but really does affect the infrastructure. Just a nice simple guide, table based on building diversity and numbers would help, very much like BS guidance on water supplies which is excellent and works to diversify pipe sizes. 
Chris Pearson
2045 Posts
Dale Green:
How does that ever stack up on Christmas Day, every oven full on for hours, heating on full wack etc.

Two points:

(1) Once an oven is up to temperature, the thermostat ensures intermittent drawing of current. Not all of the rings will be on for the same time, or even at the same time and again, thermostats ensure that loads are intermittent.

(2) Ignoring 2020, some of the flats will be full due to visiting family; others will be empty 'cos the household is away visiting family.

Swings and roundabouts - given sufficient separate users, it all averages out in the end.

2851 Posts
The difference is that the DNOs have an accumulation of network experience to draw on, and the cost/benefit trade off does not favour overdoing it when you have thousands of miles of cables, as opposed to a a few tens of metres. It may feel odd to put up to 70 houses per phase on a 500kW substation, but the practical result is that it works even if any one house can draw 100A for a short period. There is no prize for wasting energy magnetising a larger transformer core than is necessary . And if you look at the average electricity bill it bears out the lower figure.
However, activities like vehicle charging really do have the potential to stuff this up royally, being long duration heavy loads, although the network has some ability to stand short duration overloads of 100%, where short means in comparison to the heating time of a few tons of steel and copper, there is a limit of a few hours.
DNO diversity formulae may be worth a look
western power
ADMD review by university of Durham

and DNOs are wondering about changing use patterns too

The IET does not run any distribution networks at all.

441 Posts
12 Kw per flat, and the DNO allowance of 3.5Kw or less per flat are both reasonable if taken in the proper context.

I would expect an all electric flat to need a supply of at least 12 Kw to allow for simultaneous use use of a cooker, an immersion heater, and a space heater or two. More might be prudent, depending on circumstances.

However 100 such flats would not need a 1.2MW supply, nothing like it. Any one flat might well use the full 12 Kw, but they wont all use even half that much at the same time. The example of cooking Christmas dinner is a good one, not only will some flats be empty, but some will take the main meal at lunchtime and others in the evening.
The DNO allowances for average loadings often sound improbably small, but have a good record in practice.

Problems can arise when people misuse DNO style allowances for a small private developement with only a few units. The DNO might allow say 3Kw per unit over a reasonable sized estate, but that does NOT mean that 12Kw is enough for 4 flats.
Dale, I have found the ENA Engineering Recommendation P5 (Design methods for LV underground networks for new housing development) has some very useful guidance on this topic.

It provide 3 different methods for calculating the maximum demand. I have used the ADMD approach in the past to calculate the Maximum Demand for my DNO application.

The calculations allow you to include for EV chargers, different heating systems, size of apartment etc etc. 

I dont think you will get the answer in the IET Design Guide - worth buying a copy of ENA P5
Dale Green
4 Posts
Thanks everyone a few pointers there, been is some negotiations now as well so at least receiving some feedback on building diversity based on property numbers.

Looks like another spreadsheet on the way.

429 Posts
That link is interesting Mike, particularly this bit from WPD regarding the traditional 0.35 Ω

Mains cables and main overhead lines shall be protected against short circuit current.  The clearance time for faults on mains cables and main overhead lines shall be 60s or below.  The phase to earth loop impedance and the phase to neutral loop impedance shall be in accordance with ST:SD5R. Windebut is set with a maximum earth fault loop resistance of 0.19Ω (mains only) and 0.22Ω (mains and service) in order to ensure the 0.25 ohm loop impedance value for new LV circuits is satisfied.  Cut-out fuses or metering circuit breakers shall operate within 5s for faults on the terminals of the cut-out or circuit breaker.  The standard cut-out fuse rating is 80A although alternative sizes may be used. Further guidance on standard cut-out arrangements is included in ST:SD5D.

Shame they're not setting an exampIe of  how to write units after numbers..................




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